Effect of particle size on the electrocatalytic activity of platinum dispersions in carbon matrix electrodes for phosphoric acid fuel cells

The loss in electrocatalytic activity of Pt particles in carbon matrix electrodes has been experimentally and theoretically investigated as a function of Pt particle size. The measurement of the cathodic potentiostatic current transient showed that a decrease in oxygen reduction current due to carboxyl group formation, relative to the oxygen reduction current in the absence of carboxyl group, increased with a decreasing Pt particle size. This relative value is a measure of the loss in specific activity. A model describing the electrocatalytic activity loss has been proposed by introducing a new parameter, characterising the effective dead active area produced by the carboxyl group formation, relative to the total active area free of the carboxyl group. The agreement of the experimentally determined relative current decrease with the calculated relative value of the effective dead active area confirms the model.     

Theoretical and Experimental Approaches to Oxygen Reduction at Porous Composite Electrodes for Fuel Cells by Analyses of ac-Impedance Spectra and Potentiostatic Current Transients

This article covers the theoretical and experimental approaches to oxygen reduction at the porous composite electrodes for fuel cells by analyses of ac-impedance spectra and potentiostatic current transient (PCT). First, the analysis methods based upon the thin-film agglomerate model and the random packing model were introduced to theoretically calculate the ac-impedance spectra. From the results, it is suggested that the capacitance dispersion in the high frequency range is closely associated with oxygen ion migration through the electrode. The deconvolution method by discrete Fourier transform and the PCT analysis method by inverse Laplace transform were also employed to simulate the distribution function of relaxation time and the PCTs, respectively. Finally, as an example of application, in the present work, we investigated the oxygen reduction mechanism at the porous (La_0.85Sr_0.15)_0.9MnO₃ (LSM)-yittria-stabilized zirconia (YSZ) composite electrodes as a function of sintering temperature by means of the analysis methods proposed above. From the dependences of the constant phase element exponent β for ion migration and the time to reach the steady-state current t_st on the sintering temperature, the capacitance dispersion in the high frequency range was discussed in terms of the distribution of the relaxation times for ion migration, which was greatly affected by the YSZ grain size.     

A quartz crystal microbalance study of the electrochemical reaction at the interfaces of Platinum/Aqueous solution and Iron/Aqueous solution

The adsorption of the sulphate ions on the platinum electrode in sulphuric acid solution and the breakdown of the passivity in the iron electrode in the chloride-containing and chloride-free alkaline solutions have been investigated by usingin-situ electrochemical quartz crystal microbalance (EQCM) technique combined with cyclic voltammetry (CV), potentiodynamic polarization and potentiostatic current transient techniques. In the case of the platinum electrode, the combined electrogravimetric and voltammetric results are indicative of the onset of the adsorption of the sulphate ions at an applied potential between the two characteristic potentials of hydrogen desorption, above which the adsorption occurs. The adsorption of the sulphate ions was quantitatively analyzed with cyclic electrogravimetric data combined with those determined from radio tracer technique and Fourier transform infra-red (FTIR) spectroscopy by other researchers. In the case of the iron electrode, the electrogravimetric data obtained simultaneously with the potentiodynamic polarization data indicate the pitting and high-voltage breakdown of the oxide film in the chloride-containing and -free solutions, respectively. Preventing the crevice corrosion from occurring around the O-ring at the quartz crystal, the electrogravimetric curve showed that the pit initiation time measured by EQCM coincides with that time determined by current transient experiment.     

AGEING EFFECTS ON FATIGUE CRACK CLOSURE OF AN Al–Li–Zr ALLOY IN Na2SO4, SOLUTION

Abstract— Fatigue of an Al–Li–Zr alloy has been studied as a function of ageing stage by measuring crack closure in Na₂SO₄ solution with an unloading elastic compliance technique and comparing results in dry air and oxygenated solution. Anodic behaviour of peak-aged and overaged alloy specimens in the Na₂SO₄ solution has been investigated by potentiodynamic polarisation and potentiostatic current transient experiments. The Na₂SO₄ solution increased the intrinsic fatigue crack propagation (FCP) rate for the overaged specimen in dry air compared to that for the peak-aged sample. The Na₂SO₄ solution inhibited the development of crack closure for the peak-aged specimen in dry air, but aided it in the overaged condition. The result of the environmental crack closure study is discussed in terms of the more enhanced through-thickness tortuosity of the overaged specimen in the Na₂SO₄ solution when compared to that of the peak-aged specimen. The oxygen dissolved in the Na₂SO₄ solution slightly increased the environmental intrinsic FCP rate, which seems to be due to the reduced repassivation rate as compared to that in the N₂-purged solution. The anodic dissolution rate from the bare surface of the overaged specimen in the Na₂SO₄ solution was higher than that from the peak-aged sample. The difference between environmental FCP rates and crack closures for the peak-aged and overaged specimens is discussed in terms of environment-assisted crack-tip damage processes involving anodic dissolution.     

Low-threshold loss-coupled laser diode by new grating fabrication technique

In this letter, we introduce a novel fabrication method to make InGaAs absorptive first-order grating for loss-coupled distributed feedback laser diode (DFB-LD) using reactive ion etching. By this technique, it is possible to control duty rate of the first-order grating. A very-low-threshold current of 6 mA was obtained from our loss-coupled LD at the wavelength of 1.547 /spl mu/m, which was unusually high for a loss-coupled LD because of the excessive light absorption. In addition to the low-threshold current, it shows high SMSR and longitudinal single-mode yield as high as 50 dB and 90%, respectively.     

Location of Tb(III) ions in Na-Y zeolite determined by luminescence spectroscopy

The variation of the cation location and occupancy at different sites in TbNa-Y zeolites as a function of treatment temperature is monitored by luminescence spectroscopy. It is found that the spectral changes caused by thermal treatments can be qualitatively rationalized in terms of the site symmetry around the Tb(III) ion. The luminescence results demonstrate that most of the Tb(III) ions exchanged into supercages of the zeolite migrate first to sodalite cages at temperatures lower than 473 K and then migrate to hexagonal prisms between 473 and 523 K.     

Construction of an Inexpensive Subnanosecond Fluoriheter Using a Microcomputer

ABSTRACT

We describe a simple construction of a data acquisition system using an APPLE 11+ Microcomputer. The system uses a commercially available plug-in Interface board containing an analog/digital converter, a digital/analog converter, and a user dedicated 6522 Versatile Interface Adaptor (VIA). The microcomputer controls the time base and digitizes the vertical output of a sampling oscilloscope through the Interface card. Software has been developed for the data acquisition and manipulation. The system was applied to the determination of subnanosecond fluorescence lifetimes. It is pointed out that the system can be very versatile as a data acquisition and manipulation system despite its lower cost.     

Synthesis and characterization of sulfonated bromo-poly(2,6-dimethyl-1,4-phenylene oxide)-co-(2,6-diphenyl-1,4-phenylene oxide) copolymer as proton exchange membrane

Novel polymer electrolyte membranes containing the sulfonic acid groups attached on polymer backbone and side group simultaneously were synthesized. The bromo-poly(2,6-dimethyl-1,4-phenylene oxide)-co-(2,6-diphenyl-1,4-phenylene oxide) copolymer (BrcoPPO) was prepared by oxidative coupling polymerization with 2,6-dimethyl phenol, 2,6-diphenyl phenol, CuCl(I) and pyridine, and followed by bromination with bromine. Copolymer was maintained in 2,6-diphenyl phenol 10 mol% and 2,6-dimethyl phenol 90 mol%. Sulfonation of BrcoPPO (S-BrcoPPO) was carried out in a chlorobenzene solvent using chlorosulfonic acid. The polymeric membranes were cast from dimethylsulfoxide solution. The membranes were studied by nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Sorption experiments were conducted to observe the interaction of sulfonated polymers with water and methanol. S-BrcoPPO membranes exhibited proton conductivities from 2.3 × 10⁻³ to 1.4 × 10⁻² S/cm, water uptake from 7.00 to 49.43%, IEC from 0.58 to 1.38 mequiv./g, methanol permeability from 1.9 × 10⁻⁷ to 3.5 × 10⁻⁷ cm²/S.